Biomedical Engineering Reference
In-Depth Information
fraction of these voltages is lost. Electromagnetic generation is the
king at the macroscale, at the MEMS-scale they are typically limited
by the permanent magnet (PM) fabrication, as MEMS-compatible pro-
cesses cannot replicate bulk PM material properties.
Linear generators with free-sliding masses and rotational devices
are also limited by low-friction technology to minimize the mechanical
damping, which is significant at smaller sizes. Rotational devices are
the most susceptible as low-friction MEMS-based bearings are needed.
Several new technologies are being developed to overcome this limita-
tion: microball bearings (Ghalichechian et al., 2007, 2008; Waits et al.,
2007), rotating pivots (Wang et al., 2005b), and magnetic bearings
(Fernandez et al., 2000; Ghantasala et al., 2000). In addition, genera-
tors under vacuum present fewer losses due to air damping but create
the need for special packaging. Packaging is also a constraint for
devices operating in harsh conditions, such as bioimplanted applica-
tions. Fabrication techniques at the microscale are well established for
piezoelectric and electrostatic generators because of the uncomplicated
geometries they present. On the other hand, electromagnetic
generators are limited by the availability of high-performance magnets
and high-density coils at the microscale, although they are preferred
for energy generation at larger sizes. Piezoelectric and electrostatic
generators are also characterized by their relatively high voltages and
low currents, whereas electromagnetic generators provide the opposite.
Therefore, the selection of one of the transduction techniques is depen-
dent on the energy source and the desired output characteristics for
specific applications. Reliability is a point to consider because kinetic
energy harvesters use moving parts (the rotor) and no long-term stud-
ies have been undertaken. Wristwatch technology can be employed as
the mechanics of the generator are similar to those from the wristwatch
industry. Although being relatively complex mechanical devices, wrist-
watches are considered a reliable and proven technology. Therefore,
wristwatch bearing system assemblies might be used and adapted to
rotational energy harvesters to improve reliability and performance.
The output for most energy harvesters is usually a time-variant
AC signal; however, DC rectification and voltage regulation are
needed to power most electronic circuits. The forward-bias voltage for
diodes on bridge rectification circuits (
.
200 mV for Schottky diodes)
can be high for the low-voltage generated from some devices. In these
